The conventional injection molding of thermoplastic resins has a problem such that a poor appearance such as flow marks and weld lines is likely to occur in a gate and a weld during injection molding. The flow marks and weld lines are more likely to remarkably occur in the case of thermoplastic resin compositions containing a thermoplastic resin and a filler such as a colored metallic pigment or a glass fiber or inorganic filler having a high aspect ratio. Here, the “weld” means a portion produced by flows of a molten resin that merge and fuse together inside of a mold cavity. The “weld line” means only a wedged fine depression produced in the weld on a surface of a molded article, for example as shown in FIG. 2 by referential numeral 5.
Then, to improve the weld line, various techniques, methods, and products have been reported: for example, a technique for making a mold temperature high (for example, see Patent Documents 1 and 2), and a method for performing heating such that a resin melting temperature and a mold temperature are a resin deflection temperature under load or more, and molding a resin by setting the temperature of the molten resin at a predetermined temperature, and its product (for example, see Patent Document 3). Another molding method has been reported in which the surface of a mold is heated selectively to a temperature equal to or more than a heat distortion temperature, a molten resin is fed into a mold cavity, and a shear force is partially applied to the molten resin to improve flow unevenness inside of a weld (for example, see Patent Document 4).